Alignment combinators specialized for two dimensions. See
Diagrams.Align for more general alignment combinators.

The basic idea is that alignment is achieved by moving diagrams'
local origins relative to their bounding regions. For example, to
align several diagrams along their tops, we first move their local
origins to the upper edge of their bounding regions (using
e.g. map alignTop), and then put them together with their local
origins along a horizontal line (using e.g. hcat from
Diagrams.TwoD.Combinators).